Fuel cells have recently received attention as an important technique which can achieve high energy efficiency and significantly reduce emission of CO2. The type of fuel cell varies with the type of electrolyte used. For example, fuel cells for industrial application fall into four types: a phosphoric-acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), and a polymer electrolyte fuel cell (PEFC). Among them, the SOFC exhibits small intracellular resistance and is therefore known for its highest power generation efficiency in the fuel cells. In addition, the SOFC need not use any precious metal as a catalyst and therefore has the advantage that production costs can be kept down. For these reasons, the SOFC is a system widely applicable from small-scale applications, such as those for domestic use, to large-scale applications, such as a power plant, and expectations have been raised for its potential.
The FIGURE shows the structure of a general planar SOFC. As shown in the figure, a general planar SOFC includes a cell in which an electrolyte 1 made of ceramic, such as yttria-stabilized zirconia (YSZ), an anode 2 made such as of Ni/YSZ, and a cathode 3 made such as of (La, Ca)CrO3 are layered and integrated. In addition, a first support substrate 4 adjoining the anode and a second support substrate 5 adjoining the cathode are fixed to the top and bottom, respectively, of the cell. The support substrates 4 and 5 are made of metal, such as SUS. The first support substrate 4 has fuel channels 4a formed therein to serve as passages of fuel gas, while the second support substrate 5 has air channels 5a formed therein to serve as passages of air. The fuel channels 4a and the air channels 5a are formed perpendicularly to each other.
In generating electric power using the planar SOFC having the above structure, a fuel gas, such as hydrogen, town gas, natural gas, biogas or liquid fuel, is allowed to flow through the fuel channels 4a in the first support substrate 4 and concurrently air (or oxygen) is allowed to flow through the air channels 5a in the second support substrate 5. During this time, the cathode develops a reaction of ½O2+2e−→O2−, while the anode develops a reaction of H2+O2−→H2O+2e−. By these reactions, chemical energy can be converted directly into electric energy to generate electric power. To provide high-power current, an actual planar SOFC has a structure in which a number of units shown in the figure are layered.
In producing the planar SOFC, each of its component elements needs to be hermetically sealed to prevent the gases flowing through the anode and cathode from being mixed. Specifically, hermetic sealing between the support substrates, bonding of the solid electrolyte to the support substrates or hermetic sealing between solid electrolytes is necessary. For this purpose, there has been proposed a method for hermetically sealing the component elements by interlaying a sheet-shaped gasket made of inorganic material, such as mica, vermiculite or alumina, between the component elements. However, this method is simply to physically interlay the gasket between the component elements and does not involve bonding them, which may cause a tiny amount of gas leakage, resulting in poor fuel use efficiency. Therefore, consideration has been given to a method for bonding the component elements by melting using a glass material.
Each of the component elements for use in the SOFC is generally made of high-expansion metal or ceramic. Therefore, in bonding these elements using a glass material, it is necessary to conform the coefficient of thermal expansion of the glass material to those of the elements. Furthermore, the temperature range of the SOFC in which an electrochemical reaction occurs (i.e., the operating temperature range) is as high as approximately 600 to 800° C. and the SOFC is operated in this temperature range over a long period. Therefore, the glass material is required to have high thermal resistance to avoid, even when exposed to high temperatures for a long period, deterioration in hermeticity and adhesiveness due to melting of bonded portions and degradation in power generation property due to volatilization of glass components.
As a glass material having high-expansion property, a SiO2—CaO—MgO-based crystallizable glass composition is proposed which can precipitate CaO—MgO—SiO2-based crystals by thermal treatment to exhibit a high coefficient of expansion, as disclosed, for example, in Patent Literature 1. Furthermore, Patent Literature 2 discloses a SiO2—B2O2—SrO-based amorphous glass composition which has high density after being sealed and provides stable gas sealing property.